International law is a distinctive part of the general structure of international relations. In contemplating responses to a particular international situation, states usually consider relevant international laws. International law is distinct from international comity, which comprises legally nonbinding practices adopted by states for reasons of courtesy (e.g., the saluting of the flags of foreign warships at sea). In addition, the study of international law, or public international law, is distinguished from the field of conflict of laws, or private international law, which is concerned with the rules of municipal law as international lawyers term the domestic law of states of different countries where foreign elements are involved.

Public international law can be broadly defined as the body of rules, principles, and norms that govern the relationships and conduct among sovereign states, international organizations, and other entities operating within the global arena. It encompasses a wide range of areas, including the laws of war, diplomacy, human rights, international trade, and the protection of the environment, among others. Understanding the historical trajectories that have led to the current state of public international law is crucial for appreciating the complexities and nuances of this ever-evolving field.

Public international law, also known as the Law of Nations, is a set of norms aimed at regulating the interaction between the subjects of international law that participate in international relations. Public international law norms create a common framework within which the subjects of international law operate and contribute to the existence of generally stable, organized, and consistent international relations.

Definition by Individuals:

1. Oppenheim has defined international law as the "Law of Nations or International Law is the name for the body of customary and conventional rules which are considered legally binding by civilized States in their intercourse with each other.
   
   In the ninth edition of Oppenheim's book the term 'international law' has been defined as the body of rules which are legally binding on States in their intercourse with each other. These rules are primarily those which govern the relation of Organizations and, to some extent, also individuals may be subjects of rights conferred and duties imposed by international law.
    
2. J G Stake has defined international law as International law may be defined as that body of law which is composed for its greater part of the principles and rules of conduct which states feel themselves bound to observe, and therefore, do commonly observe in their relations with each other, and which includes also:
   
   1. The rules of law relating to the functioning of international institutions or organizations, their relations with each other, and their relations with states and individuals; and
   2. certain rules of law relating to individuals and non-state entities so far as the rights or duties of such individuals and non-state entities are the concern of the international community.

What does public international law apply to?
Public international law refers to subjects of international law (i.e., States and international organizations) who agree to be subject to it. The traditional doctrine of international jurisprudence did not consider individuals to be subjects of international law. However, a more modern approach holds that an individual is an individual subject to international law and the proprietor of rights and obligations within the international arena (for example, in international human rights jurisprudence, international criminal jurisprudence and international humanitarian jurisprudence). However, it is important to note that, except in the case of certain United Nations Security Council Resolutions, there is no commonly accepted binding power that can compel sovereign States to adhere to a particular body of international law.

Why is public international law important?
Public international law plays an important role because it regulates the conduct of several subjects (including States). Public international law strengthens the notion of sovereign equality between all states. It also provides predictability and equilibrium to the relations between the subjects. Moreover, it arguably limits the conduct of the subjects (such as States) by preventing them from acting in a manner inconsistent with their international commitments, which could result in international sanctions, economic reprisals, or military engagement.

What are the sources of public international law?
Meaning: 'Source', according to Oppenheim, means the ultimate origin from which the law originates. When we see a river and desire to know its source, we must go up the river until we reach a particular point where the water is oozing out naturally from the soil. That is the source of the river. Similarly, in order to find out the source of the principles of International Law we must track back to a particular point. That is the source.

In Article 38 of the Statute of the International Court of Justice, the following sources of international law are listed as having primacy before the Court of Justice: a) International conventions or treaties; b) International customary law; c) General principles of law recognized by' civilized nations; d) judicial precedents and juristic writings; and e)Ex aequo et bono(equity & good conscience).

There are 5 sources to determine public international law:
The sources of public international law are the foundational pillars from which this body of laws derives its authority and legitimacy. These sources are enshrined in Article 38 of the Statute of the International Court of Justice (ICJ), the principal judicial organ of the United Nations.

1. International Conventions or Treaties
   These are formal agreements between two or more sovereign states, or other subjects of international law, that establish a set of rules and obligations binding upon the parties involved. Treaties are the primary source of international law, as they represent the explicit consent of nations to be bound by the negotiated terms and provisions. Examples range from bilateral agreements between two countries to multilateral conventions with widespread global participation, such as the Geneva Conventions or the United Nations Convention on the Law of the Sea.
    
2. Customary International Law
   Customary international law refers to the unwritten body of legal norms that emerge from the consistent practice of states, carried out under the belief that such practice is legally obligatory. It is a source of law that develops organically through the repeated behavior and acquiescence of nations over an extended period. Customary law encompasses a wide range of areas, from the treatment of diplomatic envoys to the rules governing the use of force. Alongside treaties, it is considered a primary source of international law and is accorded significant weight by the ICJ.
    
3. General Principles of Law
   When existing treaties and customary law fail to provide a clear rule or precedent, international courts and tribunals may turn to the general principles of law recognized by civilized nations. These principles represent the common legal axioms and concepts that underpin the domestic legal systems of various countries, such as principles of fairness, due process, and good faith. While the precise scope and application of these principles are subject to scholarly debate, they serve as a supplementary source of international law, filling gaps where the primary sources are silent or ambiguous.
    
4. Judicial Decisions and Scholarly Writings
   While not direct sources of law in themselves, judicial decisions of international courts and tribunals, as well as the writings of highly qualified publicists (legal scholars), are considered subsidiary means for determining the rules of international law. The ICJ Statute acknowledges their importance as interpretive tools, providing guidance on the application and analysis of the primary sources. However, it is important to note that the doctrine of binding precedent, as practiced in domestic legal systems, does not strictly apply in international law.
    
5. Ex Aequo et Bono (Equity and Good Conscience)
   This Latin maxim, enshrined in Article 38(2) of the ICJ Statute, allows the Court to render decisions based on principles of equity and good conscience when the parties to a dispute have explicitly consented to such an approach. In essence, it permits the Court to go beyond the strict confines of codified law and treaties, and instead rely on considerations of fairness, reasonableness, and the circumstances of the case at hand. However, this authority can only be exercised with the mutual agreement of all parties involved, underscoring the consensual nature of international law.

Origins and Early Development:
The origins of public international law can be traced back to ancient civilizations and their early legal systems. The Code of Hammurabi, one of the earliest known written legal codes from ancient Mesopotamia, established principles of justice and fairness that laid the foundation for future legal developments. Ancient Greek and Roman philosophers, such as Aristotle and Cicero, also made significant contributions to the conceptualization of natural law and the idea of a universal legal order.

However, the concept of international law, as we understand it today, emerged more prominently during the medieval period. Thinkers like Francisco de Vitoria and Hugo Grotius played a pivotal role in developing the principles of sovereignty, equality of states, and the law of nations. Grotius, in particular, is considered the father of modern international law for his influential work, "De Jure Belli ac Pacis" (On the Law of War and Peace), published in 1625. This treatise established a framework for the conduct of war and the peaceful resolution of conflicts between nations, drawing upon principles of natural law and the concept of a universal legal order.

Developments in the 19th and Early 20th Centuries:
As the 19th century progressed, the concept of the nation-state solidified, and the intricate web of international relations grew increasingly complex. In this evolving landscape, the need for a more structured and codified system of international law became glaringly apparent. The old order, characterized by shifting alliances and the pursuit of national interests through sheer power dynamics, was gradually giving way to a new paradigm – one that sought to establish a framework of rules and collective mechanisms to govern the conduct of nations.

The Concert of Europe, established in the wake of the Napoleonic Wars, represented one of the earliest attempts to create such a framework. Born out of the Congress of Vienna in 1815, this concert of major powers aimed to strike a delicate balance of power and provide a diplomatic platform for resolving international disputes through collective action and consultation. While imperfect and ultimately short-lived, the Concert of Europe laid the groundwork for future efforts to institutionalize international cooperation and conflict resolution.

Parallel to these developments, the 19th and early 20th centuries witnessed a growing momentum to codify the laws governing armed conflicts and the treatment of combatants and civilians. The Lieber Code, issued during the tumultuous American Civil War in 1863, was a pioneering effort in this regard. Drafted by legal scholar Francis Lieber, the code established groundbreaking principles for the humane treatment of prisoners of war and the protection of civilian populations, sowing the seeds for future developments in the laws of armed conflict.

The Hague Peace Conferences of 1899 and 1907 marked a watershed moment in the codification of international law. These unprecedented gatherings of nations resulted in the establishment of the Permanent Court of Arbitration, one of the earliest institutional mechanisms for the peaceful settlement of international disputes. Moreover, the conferences yielded a series of conventions that sought to regulate the conduct of war and affirm the principles of neutrality, culminating in the adoption of the landmark Geneva Conventions.

The Geneva Conventions, which continue to be updated and refined to this day, represented a monumental achievement in mitigating the suffering of combatants and civilians caught in the throes of armed conflict. These conventions established fundamental protections and guidelines for the treatment of wounded soldiers, prisoners of war, and civilian populations, reflecting the growing recognition that the brutalities of war must be tempered by a shared commitment to human dignity and the principles of international humanitarian law.

As the world entered the 20th century, the foundations of a more structured and rules-based international order were taking shape. The efforts of this era, driven by the collective wisdom of nations and the pioneering work of legal scholars and diplomats, laid the groundwork for the subsequent development of a comprehensive system of international law, one that would be tested and refined by the cataclysmic events of the World Wars and the birth of new global institutions.

The Post-World War Era and the Establishment of the United Nations:
The devastating consequences of World War I and World War II highlighted the need for a more robust and comprehensive system of international law and institutions. The League of Nations, established in 1919 after World War I, was an early attempt at creating an international organization to promote peace and security. However, its limitations and lack of enforcement mechanisms ultimately led to its failure.

The Nuremberg Trials, held in the aftermath of World War II, marked a significant milestone in the development of international criminal law. These trials established the principle of individual criminal responsibility for crimes against peace, war crimes, and crimes against humanity, paving the way for the subsequent development of international human rights law. The establishment of the United Nations (UN) in 1945 was a seminal event in the evolution of public international law. The UN Charter enshrined fundamental principles such as the sovereign equality of states, the prohibition of the use of force, and the peaceful settlement of disputes. Additionally, the creation of the International Court of Justice (ICJ) provided a permanent judicial body for the settlement of international disputes through legal means.

Contemporary Developments and Challenges:
In the post-World War II era, the proliferation of international organizations and treaties has further shaped the landscape of public international law. The World Trade Organization (WTO), established in 1995, has played a pivotal role in developing and enforcing international trade law, while the International Criminal Court (ICC) and the Rome Statute of 1998 have advanced the pursuit of international criminal justice.

However, public international law continues to face numerous challenges in the contemporary era. Issues such as human rights violations, humanitarian interventions, environmental degradation, and the role of non-state actors in the global arena have posed significant tests for the existing legal framework.

The debate over the legality and legitimacy of humanitarian interventions, where military force is used to protect civilian populations from grave human rights abuses, has highlighted the tension between the principles of state sovereignty and the responsibility to protect. The NATO intervention in Kosovo in 1999, for instance, sparked intense debates about the permissibility of such actions under international law.

Similarly, the need for a comprehensive international environmental law regime to address global challenges like climate change and biodiversity loss has gained increasing urgency. The adoption of the Paris Agreement in 2015 and the ongoing negotiations on issues such as carbon emissions, deforestation, and sustainable development have underscored the importance of international cooperation and legal frameworks in tackling these pressing challenges.

The growing influence of non-state actors, such as multinational corporations, non-governmental organizations (NGOs), and terrorist groups, has also posed challenges for the traditional state- centric model of international law. Issues of corporate accountability, transnational advocacy networks, and the regulation of non-state armed groups have necessitated the adaptation and evolution of international legal frameworks.


Conclusion
The path of public international law's development over the centuries has been intricate and ever evolving, shaped by a myriad of landmark events, groundbreaking treaties, and the profound influence of pioneering thinkers. From its ancestral roots in the ancient civilizations to the modern-day trials posed by globalization and emerging transnational issues, this body of laws has displayed a remarkable ability to adapt to the shifting sands of global relations and governance.

As our world becomes increasingly interconnected, with challenges that permeate national boundaries, the significance of public international law in fostering peace, stability, cooperation, and an international rules-based order cannot be overstated. Although the obstacles we face are daunting – from egregious human rights violations and humanitarian crises to environmental degradation and the growing clout of non-state actors – the historical arc of public international law bears testimony to its resilience and capacity for transformation.

Delving into the historical context and the multitudinous factors that have influenced the evolution of public international law across the centuries offers invaluable insights into its core tenets, strengths, shortcomings, and potential to tackle the intricate global issues of our times. This historical lens illuminates the political, social, and philosophical forces that have sculpted this body of laws, from the ancient notions of natural law and justice to the modern precepts of state sovereignty and human rights.

Public international law's evolution is an ongoing journey, shaped by the collective endeavors of nations, international organizations, and the global community, striving to create a more just, equitable, and peaceful world order. As new frontiers emerge – cybersecurity, digital governance, space exploration, and the regulation of emerging technologies – international law must continue to evolve and adapt, drawing upon its rich historical foundations while embracing innovative solutions.

Studying the historical development of public international law transcends mere academic pursuit; it lays an indispensable foundation for addressing the myriad global issues that demand multilateral cooperation and adherence to a shared set of norms and principles. By comprehending the past, we can better navigate the present and chart a future where international law serves as a robust framework for upholding human rights, environmental sustainability, global economic cooperation, and the peaceful resolution of conflicts.

In an era defined by interdependence and the increasingly porous boundaries between domestic and international affairs, the importance of a strong, effective, and widely accepted system of public international law cannot be overstated. As we confront the challenges of the 21st century, the lessons gleaned from this field's historical evolution will continue to guide and inform our efforts to build a more just, secure, and prosperous global community.

Reference:

• Statute of International Court of Justice: https://eprints.soas.ac.uk/5433/1/Introduction.pdf
• https://eprints.soas.ac.uk/5433/1/Introduction.pdf
• https://legal.un.org/ilc/documentation/english/ASIL_1947_study.pdf
• https://www.tandfonline.com/doi/full/10.1080/23801883.2023.2183878
• https://www.law.cornell.edu/wex/customary_international_law
• https://law.duke.edu
• https://guides.loc.gov/public-international-law
• Book: International law & Human Right by H.O Agarwal